A window should open like the one below – this is the monitor app and you need to leave it open when you want to monitor with a webcam.

Ok… what about the multiple camera thing? Simple, press the gear button in the top-right hand corner. The settings windows will open within the monitor app. Go to the Cameras drop-down, and if you have multiple cameras available on that PC they will show-up in the drop-down as shown below.

Repeat this entire set of instructions for every camera you want to use, and assign them each their own monitor app.